The present invention relates to a process for the production of adipic acid, and particularly to a process for producing adipic acid by oxidizing hydroxycaproic acid or xcex5-caprolactone using a specific catalyst.
Adipic acid is used as a raw material of various organic chemical products such as nylon 6,6, a raw material of urethane (1,6-hexamethylendiol) and plasticizers.
Hitherto, adipic acid has generally been produced by the oxidation, with nitric acid, of a mixture of cyclohexanol and cyclohexanone, both of which are obtained by the liquid-phase oxidation of cyclohexane, the mixture being henceforth referred to as KA oil. Although the production process by the oxidation of KA oil with nitric acid provides adipic acid in a high yield, it has a problem about the disposal of nitrogen oxides since it produces as by-products nitrogen oxides such as nitrous oxide N2O, which has much effect on global warming. For example, the oxidation of cyclohexanol with nitric acid produces nitrous oxide as by-products in accordance with the following formula:
C6H11OH+2HNO3xe2x86x92HOOCxe2x80x94(CH2)4xe2x80x94COOH+N2O+2H2O
Processes of directly oxidizing cyclohexanol or cyclohexanone with air in place of nitric acid, which have been under development, are not very satisfactory in the yield of the product.
Moreover, a process for obtaining adipic acid by carbonylation of butadiene with carbon monoxide has been under development, but it is problematic in that it needs reaction conditions including high pressure or it can not achieve the sufficient yield of the product.
Incidentally, cyclohexanone and cyclohexanol are now produced by liquid-phase oxidation of cyclohexane in an industrial scale. The production process, however, gives cyclohexanone and cyclohexanol in a total yield of from about 70% to about 90% and also produces a by-product in a total yield of from about 10% to about 30%, which contains adipic acid, hydroxycaproic acid, xcex5-caprolactone and the like in relatively high concentrations. For example, when cyclohexane is oxidized with air using a cobalt salt as a catalyst, the acid waste water provided as a by-product contains 5% by weight to 15% by weight of adipic acid, 5% by weight to 10% by weight of hydroxycaproic acid, 0.1% by weight to 0.5% by weight of xcex5-caprolactone and 0.1% by weight to 0.5% by weight of adipic ester. Although recovering adipic acid from the by-produced acid waste water is considered to be desirable from the viewpoints of efficiency of resources utilization and reduction of environmental load, hydroxycaproic acid and xcex5-caprolactone are disposed by a method of incineration or the like with little recovery due to their little demand.
Under such circumstances, the present inventors diligently studied for the purpose of finding a process for producing adipic acid without using nitric acid as a raw material, which becomes the main factor responsible for the by-production of nitrogen oxides, and without using any particularly high-pressure conditions. As a result, the present inventors have found that when hydroxycaproic acid and/or xcex5-caprolactone is oxidized using a specific catalyst, adipic acid can be produced efficiently with no nitric acid or no particularly high-pressure conditions and also found that by this process, hydroxycaproic acid, xcex5-caprolactone and the like obtained as by-products and previously, mainly incinerated to be disposed can be used effectively. Also, the present inventors have found that such a process for producing adipic acid satisfies all the foregoing objects by oxidizing even the hydroxycaproic acid, xcex5-caprolactone and the like, that are the by-products in producing cyclohexanone and cyclohexanol by the oxidation of cyclohexane, with the specific catalyst. After such findings, the present inventors have completed the present invention.
Thus, the present invention provides a process for producing adipic acid which comprises a step of oxidizing hydroxycaproic acid and/or xcex5-caprolactone with oxygen or oxygen-containing gas using a metal in the platinum group as a catalyst.
The present invention uses hydroxycaproic acid and/or xcex5-caprolactone as raw materials.
In a state of an aqueous solution, xcex5-caprolactone is hydrolyzed to form hydroxycaproic acid and hydroxycaproic acid is cyclodehydrated to form xcex5-caprolactone so that both compounds gradually go toward an equilibrium state thereof in an aqueous solution with time. In the present invention, as a raw material may be used either hydroxycaproic acid alone or xcex5-caprolactone alone. However, whichever compound is used, a mixture of both compounds will be substantially used because of such an equilibrium when water is used as a solvent.
The hydroxycaproic acid and/or xcex5-caprolactone to be used in the present invention may be those obtained by any method and are not particularly limited. Examples thereof include hydroxycaproic acid and xcex5-caprolactone, that are produced as by-products during the production of cyclohexanone and cyclohexanol by air oxidation of cyclohexane. In this case, adipic acid can also be produced from esters of hydroxycaproic acid contained together as impurities. The hydroxycaproic acid to be used in the present invention includes esters of hydroxycaproic acid. Examples of the esters of hydroxycaproic acid include esters produced by the dehydration of an acid such as acetic acid, oxalic acid, caproic acid or adipic acid with hyroxycaproic acid, an ester produced by the dehydration condensation of cyclohexanol and hydroxycaproic acid, and an ester produced by the dehydration condensation between two hydroxycaproic acid molecules at their carboxyl and hydroxyl groups.
In the present invention, adipic acid is produced by oxidizing a raw material, i.e. xcex5-caprolactone, hyroxycaproic acid or a mixture thereof with oxygen or an oxygen-containing gas in the presence of a catalyst comprising a metal in the platinum group, preferably in an aqueous solution state. In the case that the raw material is used in an aqueous solution state, the concentrations of the hydroxycaproic acid and/or xcex5-caprolactone in the aqueous solution may be from about 1% by weight to about 40% by weight, preferably from about 2% by weight to about 20% by weight.
As a catalyst, a metal selected from metals in the platinum group is used. Specifically, examples of the metal in the platinum group include platinum, palladium, rhodium and ruthenium and the like. At least one metal selected from these metals is applied. The catalyst may be used with being supported on a carrier or used as it is. For example, metallic fine particles such as platinum black or palladium black may be used as they are. Examples of the carrier include activated carbon, silica, alumina, titania and zeolite. Among them, activated carbon is particularly preferred. When the catalyst is used with being supported on a carrier, the supporting ratio of a metal component to the carrier may be from about 0.1% by weight to about 8% by weight, preferably from about 0.2% by weight to about 4% by weight.
An amount of the oxygen to be used in the oxidation may be about 1 time or more, preferably in the range of from about 2 times to about 20 times, in molar ratio based on the sum of xcex5-caprolactone and hydroxycaproic acid.
Specifically, the present invention may be conducted by a method in which the raw materials, i.e. xcex5-caprolactone and/or hydroxycaproic acid, are fed in a form of an aqueous solution into a tank-type reactor, a catalyst is added and then oxygen gas or an oxygen-containing gas such as air is fed thereto under stirring. Alternatively, it may be conducted by a method in which a reactor is packed with a catalyst supported on activated carbon or the like and an aqueous solution of xcex5-caprolactone and/or hydroxycaproic acid is fed together with an oxygen molecule-containing gas to the catalyst layer.
In the case of conducting the reaction in the tank-type reactor, the amount of catalyst to be used in the reaction may fall within the range of from about 0.1% to about 200%, preferably within the range of from about 0.1% to about 100%, in weight ratio based on xcex5-caprolactone and/or hydroxycaproic acid. When the reaction is conducted in a flow system using a packed layer-type reactor, xcex5-caprolactone and/or hydroxycaproic acid are fed to the catalyst layer at a space velocity (that is the weight of the raw materials fed per unit time per unit catalyst weight; WHSV) of from about 0.02 hxe2x88x921 to about 5 hxe2x88x921, preferably from about 0.05 hxe2x88x921 to about 2 hxe2x88x921.
The reaction temperature may fall within the range of from about 80xc2x0 C. to about 220xc2x0 C., preferably within the range of from about 100xc2x0 C. to about 180xc2x0 C. When the reaction temperature is lower than about 80xc2x0 C., reaction speed achieved is not satisfactory. When it exceeds about 220xc2x0 C., selectivity to adipic acid tends to decrease.
The adipic acid obtained can be recovered from the reaction mixture by crystallization. Examples of the crystallization method include a method of cooling the reaction mixture to crystallize adipic acid and a method of vaporizing water to concentrate adipic acid so as to crystallize it.
As described in detail above, according to the present invention, adipic acid can be produced without generation of nitrogen oxides. Moreover, when hydroxycaproic acid and/or xcex5-caprolactone are used as raw materials, both of which are produced as by-products in the production of cyclohexanone and cyclohexanol by oxidizing cyclohexane, the by-products can be utilized effectively. Thus, the invention has extremely great industrial value.